Disc brake systems are used on many vehicles and usually employ a rotor and a hydraulic caliper system that force brake pads against the rotor to slow the velocity of the vehicle. Often the axis of the rotor is not perfectly aligned (e.g., perpendicular) with the brake caliper. Uneven wear is thus caused on the surfaces of the rotor as the pads of the brake caliper are applied to the misaligned rotor. Similarly, improper use of brakes may cause overheating of the rotor, increasing the likelihood that the rotor's surfaces may become warped or develop high and low spots. Uneven wear, or runout as seen by the brake caliper, on the rotor's surfaces is undesirable as it causes squealing, pedal pulsation, vibration and otherwise decreases the efficiency and life of a disc brake system.
Conventionally, unevenly worn rotors are removed and turned or machined on a lathe, such as the precision bench lathes provided by Accu Industries, Inc. of Richmond, Va. Before machining, the rotor is constrained so that the axis (center line) of the rotor is parallel to the arbor (axis) of the lathe, a process known as "truing." A machining tool is located perpendicular to the spindle of the lathe. Upon securing the rotor with a centering cone and bell clamps, the machining tool is brought into contact with the surface of the rotor. Machining the rotor in this way results in a rotor whose surfaces are perpendicular, or true, to the axis of the rotor.
However, when the machined rotor is replaced on a vehicle whose axle is not perfectly perpendicular to the brake caliper, the brake caliper will be misaligned with the surface of the rotor rather than in the desired parallel relation. The surfaces of the rotor will be skewed with respect to the brake pads of the caliper, which will therefore bear harder or softer on particular portions of the rotor as the brakes are operated. As a result, the brake caliper pads will simply reintroduce into the brake rotor the same undesirable unevenness that was removed by machining, or turning, the rotors. This results in a decreased life of the rotor, decreased brake system efficiency and concomitant increased costs in maintaining the vehicle's brake system. Potentially, this problem could be solved by simply assuring that the calipers are properly aligned with the vehicle's axis. Such precision is, however, difficult to achieve and costly to implement.
For brake rotors with uneven surfaces, the typical solution to these problems was to tilt the rotor about its axis so that its surfaces were parallel with the machining tool by providing shims between the holding clamps and the rotor. Essentially, the rotor is shimmed out so that its position on the lathe in relation to the machining tool is exactly the same as the rotor's position on the vehicle's axis in relation to the brake calipers. Numerous deficiencies are inherent in this approach, including the potential for misjudging the size of shim needed, with the result that it may cause greater or lesser skew in the rotor than is required to compensate for the runout, or the shim may loosen and become a dangerous flying object should the turning lathe cast it off. Similarly, the shim could be overly large and overlap into the area being resurfaced. Moreover, shims decrease the efficiency of the clamps, and as the lathe turns the unsatisfactorily constrained workpiece and a cutting or machining tool is applied, the workpiece likely will move out of its skewed position in which runout can be compensated. Consequently, if the workpiece shifts during machining runout is not compensated or, if the shift is sufficiently great, the workpiece could be dangerously ejected from the lathe.
In any event, shims, being merely an imprecise stop-gap for compensating for uneven surfaces, require expenditure of significant trial and error time and labor in selecting for each brake rotor the correctly sized shim. Moreover, because shims do not provide any way continuously to vary the amount of unevenness or runout that is being compensated, the user may have to try various shim sizes before the uneven surface of a brake rotor is fully compensated. In short, placing shims correctly into the clamp holding the brake rotors is an awkward, time consuming and unsafe procedure.